Balter was tasked with taking samples of concrete from every fifth truck that arrived at the construction site. The samples, taken before they were poured, were in cylinders and “cured,” or allowed to harden, according to the report.
They were left to set for varying lengths of time before they underwent compressive testing to determine the strength.
Concrete issues at the Silver Spring Transit Center
ARCHIVES | See past Washington Post coverage of the Silver Spring Transit Center.
How should the problem be remedied and who should foot the bill?
The facility was criticized on social media following the release of a report deeming it unusable.
See the consultant's report: The opening of the $112 million bus-and-train hub, already two years behind schedule, is on hold indefinitely.
It isn’t clear how Balter ended up with test results showing that the concrete was stronger than required by design specifications while KCE’s subsequent testing of the finished slabs showed them to be below contract requirements. But the KCE report offered a couple of possible scenarios.
The hardening of concrete is driven by a chemical reaction between cement and water, a process known as hydration. KCE said it was possible that the samples taken by Balter were cured in a “more hospitable environment” than the existing slabs themselves, possibly at temperatures between 60 and 80 degrees, accelerating hydration and allowing hardening to occur more rapidly.
At the same time, according to KCE, portions of the concrete were poured in cold weather, which slows hydration. The report suggests that the concrete poured into place may not have been properly warmed. The difference in concrete strength, KCE said, is “suggestive of insufficient and inconsistent quality control in cold weather conditions.”
Also, KCE said it is impossible to determine from documentation submitted by Balter how much water was added to the transit center concrete. Moreover, when KCE examined Balter’s analysis of cylinder samples, they all showed the same water-cement ratio — which is statistically impossible.
Balter’s test results showed that the concrete was stronger than the contractually required 8,000 pounds per square inch. But samples pulled from the site by KCE showed a strength of 6,970 psi on five of 18 major pours on the second and third levels of the building. Overall, portions of the building “contain unacceptable concrete” that will have to be strengthened with new overlays, the consultants said.